Abstract
Megathrust earthquakes and their associated tsunamis cause some of the worst natural disasters. In addition to earthquakes, a wide range of slip behaviors are present at subduction zones, including slow earthquakes that span multiple orders of spatial and temporal scales. Understanding these events may shed light on the stress or strength conditions of the megathrust fault. Out of all types of slow earthquakes, very low frequency earthquakes (VLFEs) are most enigmatic because they are difficult to detect reliably, and the physical nature of VLFEs are poorly understood. Here we show three VLFEs in Cascadia that were dynamically triggered by a 2009 Mw 6.9 Canal de Ballenas earthquake in the Gulf of California. The VLFEs likely locate in between the seismogenic zone and the Cascadia episodic tremor and slip (ETS) zone, including one event with a moment magnitude of 5.7. This is the largest VLFE reported to date, causing clear geodetic signals. Our results show that the Cascadia megathrust fault might slip rapidly at some spots in this gap zone, and such a permissible slip behavior has direct seismic hazard implications for coastal communities and perhaps further inland. Further, the observed seismic sources may represent a new class of slip events, whose characteristics do not fit current understandings of slow or regular earthquakes.
Plain Language Summary
Megathrust earthquakes and their associated tsunamis pose significant hazards in Cascadia. In addition to earthquakes, episodic tremor and slip events have been discovered in the region at depths of 30–50 km, 10–15 km below the seismogenic zone. The expected slip behavior between the seismogenic and tremor zones remains unknown, leaving the rupture extents of future megathrust earthquakes unclear. We detect and locate three very low frequency earthquakes (VLFEs) near this gap region, including one with a moment magnitude of 5.7. This is the largest VLFE reported to date, and its detection not only shows that the fault slip behaviors in this gap zone can be highly heterogeneous but also challenges current understanding of slow earthquake physics.
Key Points
Three large very low frequency earthquakes (VLFEs) in Cascadia were dynamically triggered by teleseismic waves
These VLFEs likely occurred in between the seismogenic and tremor zones
One VLFE has a moment magnitude of 5.7, the largest VLFE detected to date and the first detected geodetically
